`--no-builtin-tools` / `-nbt`: disable built-in tools but keep extension and custom tools

[quangdang46]

--no-builtin-tools / -nbt: disable built-in tools but keep extension/custom tools

Summary

Add a separate --no-builtin-tools (-nbt) flag, distinct from a hypothetical --no-tools. The flag disables only the built-in tools (read, write, edit, bash, grep, find, ls, etc.) while leaving extension-provided tools, MCP tools, and custom tools enabled.

Reference: pi-mono Tool Options — distinguishes --no-builtin-tools from --no-tools.

Why

• Running an extension-only or MCP-only session is currently awkward because there is no clean way to silence built-ins without also silencing what the user actually wants.
• Combining --no-builtin-tools with --tools <list> (already a planned/existing flag) lets users build a deterministic minimum tool surface, useful for security demos and conformance testing.

Current state in jcode

• src/cli/args.rs has --tools <list> style options but no --no-builtin-tools flag.
• The tool registry that needs to gate built-ins lives in src/tool/mod.rs and crates/jcode-tool-core/.
• Extension-registered tools (depends on issue TypeScript/QuickJS extension runtime for user-defined tools, commands, and hooks #3) and MCP tools (in src/mcp/) flow through a different registration path.

Implementation checklist

1. CLI

• Add no_builtin_tools: bool (short -nbt, global) to Args in src/cli/args.rs.

2. Registry

• In whatever construction step builds the active tool list, skip built-ins when args.no_builtin_tools is true.
• Built-ins set: read, write, edit, multiedit, apply_patch, bash, grep, find/glob, ls, agentgrep, webfetch, websearch, todo, memory, skill, task, communicate (mark which of these are user-facing built-ins vs internal — exclude task / communicate / todo from the gate since they are infrastructure tools).

3. Combine with explicit --tools

• When both --no-builtin-tools and --tools are set, the explicit --tools list wins for the named built-ins (so users can re-enable individual ones without losing extensions).

Testing

• Unit test in src/agent_tests.rs: starting a session with --no-builtin-tools exposes zero built-in tools to the model.
• Combined with --tools read: only read is exposed.
• MCP tools and extension tools (mocked) remain enabled.

Acceptance criteria

• jcode --no-builtin-tools "..." runs and the model sees only extension/MCP/custom tools.
• jcode --no-builtin-tools --tools read "..." runs and the model sees read plus non-built-ins.
• --help and README document the precedence.

Cross-references

• Issue TypeScript/QuickJS extension runtime for user-defined tools, commands, and hooks #3 — extension runtime (this flag only makes sense once TypeScript/QuickJS extension runtime for user-defined tools, commands, and hooks #3 lands).
• Issue Interactive /settings UI for config inspection and edits #7 — /settings UI should mention this gate.

Reference

• pi-mono: Tool Options.

[quangdang46]

Implementation Plan

Scope

--no-builtin-tools (alias -nbt) disables jcode's built-in tool registry but keeps user-installed extension tools and MCP server tools.

Files

• src/cli/args.rs — no_builtin_tools: bool flag.
• src/cli/startup.rs — translate to JCODE_NO_BUILTIN_TOOLS=1 env.
• src/tool/mod.rs::register_default_tools — gate built-in registrations on the env var.
• src/agent.rs — log a one-line notice at session start when active.

Behavior

• Built-ins removed: read, edit, bash, glob, grep, websearch, browser, todo, skill, memory.
• Kept: any tool registered by an MCP server or extension (issue TypeScript/QuickJS extension runtime for user-defined tools, commands, and hooks #3 once landed).
• For now (extensions not yet landed), this flag effectively means "MCP-only" — useful for sandbox testing.

Tests

• no_builtin_tools_excludes_default_registry
• no_builtin_tools_keeps_mcp_tools
• flag_translates_to_env_var

Estimated scope: S — ~50 LOC + 3 tests.